1. Field of the Invention
The present invention relates to a modular hip prosthesis system wherein various component parts of an implant can be selected during the surgical procedure so that the implant can be custom fitted to a patient.
2. General Background
Increasingly, surgeons want to be able to custom fit femoral prostheses to patients. Instead of having to choose a properly sized prosthesis from a group of pre-formed implants, it would be advantageous to have a basic design which can be modified with various component parts. This would eliminate the need to maintain a large inventory and would provide better fitting implants.
Custom fitted implants are particularly important in revision cases where an implant has to be removed and replaced since old cement must be removed and bone resorption occurs in many cases. Unpredictable, proximal and/or distal bone loss or deformity often occurs which must be accommodated by the replacement prosthesis.
For initial implants, basic variations in patient anatomy are often confronted by the surgeon. Some patients have femoral necks that can be significantly longer or shorter than others. Cortical shaft defects, which are not uncommon, are not discovered until during operation. Variations in intramedullary canal diameter can also occur, which if not accompanied by a properly sized implant, can result in distal toggle.
In short, it is difficult, if not impossible, to predict the exact shape of a hip implant which is suitable for all patients. Since it is impractical and would be prohibitively expensive to maintain an inventory of implants for most patients, compromises must sometimes be made in supplying implants which fit reasonably well but could be improved upon.
Several hip prostheses are known which are formed of replaceable or interchangeable component parts.
U.S. Pat. No. 3,641,590 entitled "Acetabular Replacement Prosthesis and Method of Assembling" issued to Michele discloses a selective individualized technique for acetabulum socket replacement per se, or in conjunction with a hip replacement prosthesis (referring to the Michele U.S. Pat. No. 3,228,393) for a total hip replacement, designed for all ages including the very young. A selective anchorage for a cup prosthesis of a size selected from the limited number of differently sized cups is made available. Anchorage of the acetabular socket replacement conforms to variations in dimensions, shapes and positions of the (medullary) canals of the acetabulum pelvis of the individual patient and includes at least two elongated and convergent or divergent fasteners.
A removable collar of low modulus of elasticity material is shown in U.S. Pat. No. 4,012,796 entitled "Interpositioning Collar For Prosthetic Bone Insert" issued to Weisman et al. The collar is interpositioned between a collar of a metal prosthetic hip stem implanted in the intramedullary canal of the femur and the adjacent calcar or outer edge of the bone. A flange depends from the insert between the upper portion of the stem and the inner wall of the bone. The interpositioned collar is either a full elongated tapered 0-shape or it is open on one side of a tapered U-shape.
U.S. Pat. No. 4,404,691 entitled "Modular Prosthesis Assembly" issued to Buning et al., provides a modular hip prosthesis assembly for replacement of at least part of a joint and part of a bone shaft including a mounting component provided with a connection portion and at least two joint components of similar shape but different dimensions and which can be connected alternatively to the mounting component, each of the joint components having an engagement portion and a connection part adapted for connection to the connection portion of the mounting component, the joint components each providing part of a bone shaft and part of a joint which can cooperate with an appropriate part of a natural or artificial joint.
U.S. Pat. No. 4,578,081 entitled "Bone Prosthesis" issued to Harder et al., discloses a bone prosthesis comprising at least one joint component replacing a natural joint half, which is provided with a shank adapted to be connected to the bone, wherein a set of joint components is provided, and the shank is designed as a bone replacement member, with a connection portion provided adapted to be connected to the bone at one end and at the other end to the shank. One of the components is a hip prosthesis with a rounded head and a hollowed hip component that connects to elongated mounting components. In another hip prosthesis embodiment, a neck with a cone shape receives a suitable joint head with an inner cone.
A femoral component for hip prosthesis is shown in U.S. Pat. No. 4,608,055 issued to Morrey et al., the prosthesis disclosed in the '055 patent includes a stem portion and a combined integral head and neck portion. The stem portion includes a proximal portion and a distal portion which are angularly related with respect to one another and with the proximal portion including a recess formed therein for receipt of the tapered portion of the head and neck component. The head and neck component includes a substantially part spherical head portion attached to a neck portion and a tapered portion angularly attached to the neck portion via a basalar neck portion with the tapered portion being adapted to be permanently inserted into the recess portion of the proximal end of the above described stem portion. The stem portion includes a plurality of recesses on the periphery and longitudinal extent thereof for receiving fiber metal pads which are provided to allow boney ingrowth therein in order to retain the femoral component permanently installed in the proximal end of the femur.
In U.S. Pat. No. 4,676,797 entitled "Unit For Resection Prosthesis", a resection prosthesis assembly unit includes a head member, an end member and an intermediate member between the head and end members, of which one member is provided with a conical pin and another member is provided with a conical pin and another member is provided with a conical bore. The latter two members are provided with respective first surfaces extends transversely to an insertion direction and which face one another and are spaced apart to define a recess when the two members are connected together. One of the two members is further provided with a second surface extending in the insertion direction, and the recess being provided to receive a wedge insertable into the recess to bear against the first surfaces for forcing the two members apart while the forces exerted by the wedge are absorbed by the first surfaces and the wedge is guided by the second surface.
Stem extensions and/or augmentation are provided which allows the length of the femoral shaft to be augmented with extensions that lock with the primary shaft using a tapered attachment. This attachment allows the surgeon to extend the stem length beyond any cortical shaft defect he may encounter intraoperatively. Additionally, end fitting caps attached in a similar fashion could allow the surgeon to tailor the prosthesis tip to the canal diameter thereby reducing distal toggle. The prosthesis bodies could be provided in a variety of inclinations of the access of the modular head and or neck module with respect to the longitudinal axis of the prosthesis body and the stem. The surgeon could intraoperatively select the most appropriate amount of version (anteversion or retroversion) to restore joint stability. Increase attainable neck length variation is provided for allowing "fitting" to patients with very short femoral neck (i.e. CDH, small females) or very long femoral necks (i.e. to components for proximal bone loss). Head neck assemblies could also incorporate a collar or augmented extension to convert the standard stem body into a proximal 1/3 replacement or a revision stem. A separate set of head neck assemblies could be offered to allow the surgeon to tailor the neck shaft angle of the assembled device as well.